The present invention relates to an encapsulated barrier film structure and a method of making the same. More specifically, the present invention relates to a thermoplastic, multilayer packaging film having a barrier layer consisting of a thermally sensitive polymer material, preferably ethylene vinyl alcohol. The material acts to prevent the passage of gases such as, for example, oxygen and/or water vapor, from one side of the film to the other. The barrier material is encapsulated by one or more layers of a relatively thermally stable material that acts to protect the barrier material from high temperatures and/or long residence times present during coextrusion, lamination or coating that may destroy or otherwise degrade the barrier material. The one or more thermally stable encapsulating layers aid in binding the thermally sensitive barrier layer to outer layers having relatively higher melt and/or extrusion temperatures thereby maintaining optical clarity of the film produced therefrom. In addition, the present invention relates to using an acid terpolymer as an adhesive to bind the thermally sensitive barrier core material to high temperature outer layers while eliminating clarity problems associated with using other adhesives.
It is, of course, generally known to utilize a polymeric material as a barrier material in films to prevent the passage of molecules such as, for example, gases and water vapor. Films may have these barrier properties to protect foods or other gas-sensitive materials that may be contained within bags or other containers made from the films. In particular, food articles are subject to the deleterious effects of gases and water vapors.
A known film structure that prevents the passage of molecules therethrough uses polyvinylidene chloride (xe2x80x9cPVdCxe2x80x9d) or polyvinylidene chloride/methyl acrylate copolymer (xe2x80x9cPVdC/MAxe2x80x9d), commonly known as MA-Saran and manufactured by Dow Chemical Company. These barriers are generally useful in preventing molecules such as oxygen from passing therethrough but are fairly unstable at the high temperatures needed to produce many multilayer films from a molten resin. Typically, PVdC degrades at high temperatures forming polyenes reducing the optical clarity of films made therefrom. A suitable, albeit more costly, substitute for MA-Saran is ethylene vinyl alcohol copolymer (xe2x80x9cEVOHxe2x80x9d).
Another film that is commonly used as a barrier layer, especially for food products such as cheese, is a PVdC coated oriented polypropylene (xe2x80x9cOPPxe2x80x9d) layer. Structures made using this barrier material have good barrier characteristics. Specifically, barrier layers of PVdC coated OPP adequately restrict the movement of oxygen molecules or water vapor through packaging made therefrom. However, PVdC coated OPP is cost prohibitive.
Generally, EVOH is thermally stable at higher temperatures relative to PVdC or MA-Saran. However, EVOH is still sensitive to high temperatures, particularly when adhered to a layer of polyethylene (xe2x80x9cPExe2x80x9d) having maleic anhydride functional groups. While EVOH may be extruded at higher temperatures relative to PVdC, the temperature of extrusion may still be too low for coextrusion with other layers that require very high temperatures for adequate melting and/or adhesion to lamination or coating substrates.
Typical methods of coextrusion generally entail feeding the barrier material and adhesive resins into a feedblock where they are arranged into an xe2x80x9cA/B/Axe2x80x9d configuration prior to extrusion through a die. The adhesive layers must be compatible with the barrier layer as well as the substrates that are being laminated or coated. Further, the adhesive layers must be at or greater than 600xc2x0 F. to adequately adhere to the substrates. However, this adhesive layer melt temperature requires that the downstream hardware (such as, for example, the feedblock and/or the die) be at or greater than 600xc2x0 F. as well. Many barrier materials, including, especially, EVOH, readily degrade when exposed to temperatures greater than about 450xc2x0 F. for extended periods of time. Due to this degradation, as well as the extensive reaction that may occur between the barrier material and the adhesive layer at the layer interface, the resulting extrudate may have clarity or other problems. For example, EVOH reacts with maleic anhydride, a typical adhesive layer used with EVOH, to produce a xe2x80x9cground glassxe2x80x9d appearance when coextruded at high temperatures for extended periods of time.
A known process of coextruding and laminating heat sensitive materials is described in U.S. Pat. Nos. 5,106,562, 5,108,844, 5,190,711 and 5,236,642, which are hereby incorporated by reference in their entirety. Various methods are disclosed for reducing the impact of higher temperature polymeric meltstream elements on a lower temperature polymeric meltstream. The methods may include super-cooling the hotter meltstream element below the melting temperature but above the crystallization temperature, exposing one or more meltstream elements to an undesirable thermal condition for a limited period of time, and/or using one or more layers as a heat sink via encapsulation.
Specifically, these patents describe methods of encapsulating one film layer by another material. The ""562 and ""844 patents specifically relate to PVdC or, preferably, PVdC-MA core materials with ethylene vinyl acetate copolymer (xe2x80x9cEVAxe2x80x9d) or ethylene methyl acrylate copolymer (xe2x80x9cEMAxe2x80x9d) or blend""s thereof encapsulating the core material. The encapsulated PVdC or PVdC-MA is, therefore, protected from the high temperatures of the coextrusion process. Generally, the encapsulation method uses an encapsulator having a crosshead mandrel with a central bore to receive a first meltstream element from an extruder. A second polymeric meltstream is fed through a sleeve via an inlet passage into the encapsulator. As the second meltstream enters the encapsulator, it splits and flows around the first meltstream. Consequently, the second meltstream completely surrounds the first meltstream, thereby forming a combined meltstream. The second meltstream forms a continuous layer about the circumference of the first meltstream completely surrounding the first meltstream. Thus, the first and second meltstreams maintain their individual identities while the first meltstream is completely surrounded by and encapsulated within the second meltstream. The combined meltstream may then be fed through a transport pipe to a feedblock for coextrusion with one or more other layers to produce a multilayer film. However, these patents do not disclose other materials that may be utilized as heat sensitive barrier materials besides PVdC or PVdC-MA.
The present invention relates to encapsulating a barrier material with a first adhesive layer at the same melt temperature as the barrier material. For example, EVOH melts at less than 450xc2x0 F. and, therefore, is encapsulated by the first adhesive layer at less than 450xc2x0 F. prior to entering the feedblock. The encapsulating first adhesive layer protects the barrier material from the hot melt temperatures of a second adhesive layer, which the first adhesive material encounters within the feedblock. The first adhesive material insulates the barrier material from the much higher melt temperature of the second adhesive layer. Moreover, the first adhesive material drastically reduces the residence time of the barrier material within the hardware by keeping it away from areas of high drag and hangup, such as the internal walls of the hardware. The result is an extrudate with excellent clarity.
It is, therefore, an advantage of the present invention to provide a film structure and a method of manufacturing the same that use a relatively inexpensive barrier material to prevent the migration of gases through the film structure such as, for example, oxygen and/or water vapor, without degrading due to high heat and/or long residence times within the film producing equipment. The encapsulated EVOH barrier layer may be utilized effectively as a replacement to cost-prohibitive barrier materials such as PVdC coated OPP.
Another advantage of the present invention is to provide a film structure and a method of manufacturing the same that allow the use of a thermally sensitive barrier material to be utilized within high temperature coextrusion/lamination processes.
And, an advantage of the present invention is to provide a film structure and a method of manufacturing the same that surround the thermally sensitive barrier material with an adhesive in an encapsulating process to protect the barrier material from high temperatures that may exist in the coextrusion/lamination process.
A still further advantage of the present invention is to provide a film structure and a method of manufacturing the same that use an adhesive to encapsulate the barrier material at the melt temperature of the barrier material. Further, the adhesive layer may protect the barrier material from layers coextruded therewith or laminated thereto having higher melt temperatures. In addition, the adhesive may protect the barrier material from high temperatures of the hardware and long residence times within the hardware.
Moreover, an advantage of the present invention is to provide a film structure and a method of manufacturing the same that uses a thermally sensitive material such as EVOH as the barrier layer material to reduce costs of the film structure.
And, another advantage of the present invention is to provide a film structure and a method of manufacturing the same that eliminates interlayer leakage and deadspots in the barrier layer material flow patterns.
Additional features and advantages of the present invention are described in, and will be apparent from, the detailed description of the presently preferred embodiments and from the drawings.